“What we see is that testosterone goes down dramatically after men become fathers,” Kuwaza said, “and this could have effects on behavior – and help them focus on the duties of child-rearing and the types of things that are required in effective fathering.”

Kuzawa says the study of more than 600 men also found that men who had higher testosterone to begin with were more likely to enter stable relationships and become fathers.

But could the loss of testosterone after fatherhood have a certain unwanted effect? No, Kuzawa says enough testosterone is still produced to maintain libido.

Moving forward, Kuwaza says, diseases related to high testosterone – such as prostate cancer – the risks may be reduced after fatherhood.